1. Field of the Invention
This invention relates to liposomal formulations of compounds such as drugs. More particularly this invention relates to methods of encapsulating anesthetics in multivesicular liposomes with high efficiency and sustained in vivo rates of release.
2. Description of the Related Art
A local anesthetic's duration of action following administration is usually sufficiently long to cover the pain inflicted during most surgical procedures. However, the duration of action is not long enough to cover most post-operative pain, or pain from many invasive diagnostic procedures, or from injuries. Continuous infusion or repeated infiltration of a local anesthetic into a surgical wound, diagnostic “port” or injury site is impractical. Therefore, a sustained-release formulation of a local anesthetic would be useful for pain management, especially in view of the current trend for out-patient surgeries and emergency care centers. Desirably, such formulations are useful in trauma and diagnostic pain, as well.
Several approaches to develop sustained-release formulations of local anesthetics have been described in the literature. For example, polylactic-co-glycolic acid polymer micro spheres containing both bupivacaine and dexamethasone have produced long duration of local anesthesia. Crystalline forms of local anesthetics have also been shown to have long duration of action. Lipophilic bupivacaine free-base incorporated into the membranes of multilamellar liposomes and proton-gradient-loaded large unilamellar liposomes have shown efficacy lasting 6 to 11 hours.
Multivesicular liposomes (MVL) are being developed as a lipid-based sustained-release drug delivery system for local, regional or systemic drug delivery. Sustained release of many water-stable drugs encapsulated into MVL has been shown in animal models via intrathecal, subcutaneous, intraperitoneal and epidural routes of administration, as well as in human patients via intracerebroventricular, intrathecal, subcutaneous and epidural routes. A multicenter, randomized phase III clinical trial of a MVL formulation of cytotoxic agent cytarabine has shown that this formulation is more efficacious than free cytarabine in treating leptomengial carcinoma.
MVL are defined as liposomes containing multiple non-concentric chambers within each liposome particle, resembling a “foam-like” matrix. Such particles are to be distinguished from multilamellar vesicles (MLV), also known as a multilamellar liposome, which contain multiple concentric chambers within each liposome particle. A further distinct particle is the unilamellar vesicle (ULV), also known as a unilamellar liposome, which encloses a single internal aqueous compartment. The present invention relates to MVL. The prior art describes the preparation of MVL (Kim et al., Biochim. Biophys. Acta 728, 339-348, 1983).
Many of the cationic biologically active substances used in MVL encapsulation techniques are used as salts of monoprotic mineral acids (for example, as hydrochloride salts). The prior art has used such commonly available monoprotic mineral acid salts of cationic biologically active substances for encapsulation into liposomes without any modification into a salt of diprotic or triprotic mineral acid. The prior art has also used organic acids such as citric or glutamic acids to effect encapsulation.